<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>
  <script>
    const str = "www.lagou.com"
    const reg = /www.[a-zA-Z0-9]{0,62}.com/
    let input = prompt("请输入一个网址","www.baidu.com")
    let output = input.replace(reg,str)
    console.log(output)
  </script>
</body>
</html>